The present invention relates to a scanning apparatus of the type using a semiconductor laser as a light source and, more particularly, to a device for controlling the output beam of the semiconductor laser.
A laser beam printer, digital copier or similar image recording apparatus usually includes a scanning device which is implemented with a semiconductor laser. This type of scanning device scans a photoconductive element, or image recording element, with a laser beam having been modulated by image data, whereby a latent image is formed on the photoconductive element. It is necessary, therefore, that the semiconductor laser be stably controlled to emit a constant output beam. However, the output beam of the semiconductor laser, or laser diode (LD), is noticeably dependent on the ambient temeperature as well as to the current fed to the laser diode. To eliminate this problem, the actual output beam of the semiconductor laser may be detected as an electrical signal, e.g., a voltage and compared with a reference value to thereby control a laser driver in such a manner as to maintain the output beam constant, as has been proposed in the past. For example, Japanese Patent Publication No. 54273/1988 discloses an arrangement wherein an up-down counter is incremented when the laser output is lower than a target value or decremented when the former is higher than the latter, thereby controlling the laser output stepwise toward the target value.
The stepwise control scheme mentioned above, however, consumes a substantial period of time until the control of the laser output is completed, since the current fed to the laser diode is increased or decreased stepwise toward the target value.
With an image recording apparatus such as a laser beam printer or a digital copier, it is a common practice to develop a latent image formed on a photoconductive element, or image recording element, by use of a toner. The control of laser output is, therefore, effected with the photoconductive element being charged for protection purposes. Hence, should a substantial period of time be consumed until the end of the control as with the above-stated prior art arrangement, the toner would be unwantedly deposited on the photoconductive element during the course of the control.